Display conditions allows you to control the visibility of your app modules in the UI. You can use simple flags like isAdmin, isLoggedIn, conditions that are based on content or space properties, and combine multiple conditions together using operators like and, or and not.
Please see more details here: https://developer.atlassian.com/platform/forge/manifest-reference/display-conditions/
At this moment display conditions are only supported for Byline extension point. Support of display conditions for other extension points is coming soon.